dt-bindings: msm: dsi: Fix clock declarations

When converting from .txt to .yaml dt-binding descriptions we appear to
have missed some of the previous detail on the number and names of
permissible clocks.

Fixes: 4dbe55c9 ("dt-bindings: msm: dsi: add yaml schemas for DSI bindings")
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>